Huge Greek character blowing top (4)
I believe the answer is:
mega
'huge' is the definition.
(I've seen this before)
'greek character blowing top' is the wordplay.
'greek character' becomes 'omega' (letter from the Greek alphabet).
'blowing top' means to remove the first letter.
'omega' with its initial letter removed is 'MEGA'.
(Other definitions for mega that I've seen before include "wicked" , "Informally great or large" , "Game (anag) -- great" , "Very large (colloq.)" , "Enormous; very good" .)